The philosophy of Immanuel Kant (1724–1804) can be divided into two major branches. His theoretical philosophy, which includes metaphysics, is based on the rational understanding of the concept of nature. The second, his practical philosophy, comprising ethics and political philosophy, is based on the concept of freedom. Both of these branches have been enormously influential in the subsequent history of philosophy.

Kant’s Critique of Metaphysics

In one of history’s best-known philosophical compliments, Kant credited the work of David Hume (1711–1776) with disrupting his “dogmatic slumbers” and setting his thinking on an entirely new path. To better understand the results of this new line of thought, we should briefly consider the “dogma” in question, and Hume’s attack on it. The prevailing philosophical orthodoxy in Kant’s time was a rationalism set out by Gottfried Leibniz (1646-1716), and systematized by Christian Wolff (1679–1750). According to such rationalists, empirical knowledge based on experience is suspect because it is necessarily tied to the subjective perspectives of individuals. Because the human senses are inherently fallible, empirical investigations can never reveal how the world really is, untainted by perspective: objective knowledge of the world can be achieved only through the use of reason. Leibniz, for example, provided an account of the world derived by reason from only two basic principles, which he believed were self-evidently true.

David Hume was an exponent of empiricism, a doctrine opposed to rationalism. For empiricists, all knowledge is derived from sense experience, and, therefore, the subjective perspectives of observers can never be entirely overcome. According to this position, rationalist efforts to circumvent the senses by relying on reason alone are bound to fail. Reason can contribute to knowledge, but only by relating ideas to one another, and ideas are ultimately based on sense impressions. An independent “realm of ideas,” or access to knowledge of reality untainted by the human senses, is therefore impossible. Hume was especially effective in drawing out the skeptical implications of the empiricist position. He argued that neither personal identity nor causality could legitimately be inferred from experience. Although we might notice that some events regularly follow others, we cannot infer that one caused the other. Kant found Hume’s attack on causality particularly worrisome, because it threatened the basis of modern natural science.

In his Critique of Pure Reason, Kant lays out his response to this philosophical dispute. Kant sees the force of the skeptical objections to rationalism and therefore aims to re-establish some of the claims of reason on firmer ground. Kant agrees with the empiricists that there is no “intelligible realm” accessible only by reason, and he denies that we can gain knowledge of how the world is, independent of all experience. However, he does not conclude that all human knowledge is ultimately reducible to particular experiences. For Kant, it is possible to draw general conclusions about the sensible world by giving an account of how human understanding structures all experience. As he puts it in the Preface to the second edition of the Critique:

Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge.

Kant compares his metaphysical studies to those of Copernicus, who revolutionized the study of astronomy by accounting for the position of the observer of the celestial bodies. Analogously, Kant aims to revolutionize metaphysics by accounting for the structure of the understanding that apprehends nature. According to Kant, the sensible world has certain features that can be known a priori, not because these are features of the objects in themselves, but, rather, because they are features of human understanding. We can know a priori that all objects will exist in space and time because these are the forms of our intuition; we could not even conceive an object that exists without these forms. Similarly, all experience is structured by the categories of the understanding, such as substance and causality. On the Kantian view, human understanding becomes the legislator of nature because the “laws of nature” that we perceive in the world are put there by our understanding.

In the Critique of Pure Reason, Kant aims to show the limits of what can be known by theoretical reason, and his strategy depends on a distinction between phenomena (objects as we experience them) and noumena (objects as they exist in themselves). In one sense, Kant chastens the ambitions of reason. Because all knowledge is structured by the categories of the understanding, we must forego knowledge of things-in-themselves. However, knowledge of these categories also allows us to draw a priori generalizations about the phenomenal world. For example, we know that the natural world is governed by the principle of causality because causality is a form of knowledge. By confining his conclusions to the world of experience, Kant is able to meet the threat of Humean skepticism and put natural science on a firm foundation.

Moral Philosophy

Kant’s understanding of moral freedom and of moral principles has been central to discussions of morality from his time forward. His moral philosophy is a philosophy of freedom. Without human freedom, thought Kant, moral appraisal and moral responsibility would be impossible. Kant believes that if a person could not act otherwise, then his or her act can have no moral worth. Further, he believes that every human being is endowed with a conscience that makes him or her aware that the moral law has authority over them. Kant calls this a “fact of reason,” and he regards it as the basis for a belief in human freedom. However, Kant also believes that the entire natural world is subject to a strict Newtonian principle of causality, implying that all of our physical actions are caused by prior events, not by our free wills. How, then, can freedom and morality be possible?

In simplified terms, Kant’s answer to this problem is that although humans are subject to causality in the phenomenal realm, we are free in the noumenal realm. To make sense of this answer, it is necessary to understand Kant’s distinction between theoretical and practical reason. The Critique of Pure Reason gives an account of theoretical reason and its limits. Theoretical reason can understand the natural world through the categories of the understanding. Practical reason addresses questions of how the world ought to be and tells us our duty. It also leads humans to a concept of an ideal world, which it becomes our aim to create. However, the proper functioning of practical reason requires the existence of certain conditions, such as God, immortality of the soul, and, most importantly, free will. Because none of these is contained within the categories of the understanding, theoretical reason can know nothing about them. However, argues Kant, because theoretical reason is also incapable of disproving their existence, we are justified in accepting their existence practically. As he puts it in the preface to the second edition of the Critique of Pure Reason, Kant “had to deny knowledge in order to make room for faith.”

According to Kant, ethics, like metaphysics, is a priori, meaning that our moral duty is determined independently of empirical considerations. Kant’s ethics can therefore be contrasted with ethical views such as utilitarianism that hold that the morality of acts is derived from their consequences. In the Groundwork of the Metaphysics of Morals, Kant outlines his fundamental ethical principle, which he calls the “categorical imperative.” The moral principle is “imperative” because it commands, and it is “categorical” because it does so unconditionally, that is, irrespective of the particular inclinations and circumstances of the actor. This moral principle is given by reason and states that we may act only in such a way that the maxim of our action, i.e. the principle governing our action, could be willed as universal law. For example, one is forbidden to act on the maxim “lie whenever it provides an advantage” because such a maxim would destroy trust among humans, and with it the possibility of gaining any advantage from lying. Those who act on non-universalizable maxims are caught in a kind of practical contradiction. In another formulation of the categorical imperative, Kant specifies that we must always respect humanity in ourselves and others by treating humans always as ends in themselves, and never merely as a means.

Freedom, for Kant, is thus not the “freedom” to follow one’s inclinations. Instead, freedom implies morality, and morality implies freedom. To act on one’s inclinations or desires, even if one desires the morally correct act, is to be determined by the causal forces of nature, and therefore to be unfree or “heteronomous.” To act morally is to act “autonomously,” meaning to act according to the law that one gives oneself. It is not sufficient only to perform the acts required by morality; it is also necessary to act intentionally in accord with one’s moral duty.

Political Philosophy

Kant’s political philosophy is entwined with his moral philosophy. Political activity is ultimately governed by moral principles based on human autonomy. Therefore, in his essay “On the Common Saying: ‘This May be True in Theory, But it does not Apply in Practice,’” Kant is critical of political thinkers, such as Machiavelli, who believe that amoral or immoral means are permissible in politics. Still, although Kant argues that morality is obligatory in politics, he does not believe that people’s actual political behavior is controlled by duty.

One of the most important political acts required by duty is the establishment of a state based upon law, a Rechtsstaat. In the Doctrine of Right (the second part of the Metaphysics of Morals), Kant tells us that the only innate right is “freedom, insofar as it can coexist with the freedom of every other in accordance with a universal law.” Human freedom and dignity must be respected, and this is possible only within a constitutional state governed by law, which protects the civil rights of individuals. Kant differentiates “republics,” the kind of government he advocates, from “despotisms” according to whether their executive and legislative branches are separated from one another. When executive and legislative powers are invested in a single body, the government becomes despotic because law is no longer universal but is determined by a particular will. Direct democracies thus are inevitably despotisms because the majority oppresses the minority rather than acting according to universal law.

Kant’s emphasis on lawful government and civil rights connects him to the natural rights thinking of predecessors such as Hobbes, Locke, and Rousseau. However, Kant’s justification for the state is not limited to the justifications offered by these thinkers. Kant does not argue merely that individuals enter the state or social contract for prudential reasons, because their interests are best served by the state, but also that we have an obligation to respect human freedom, and this requires us to create a Rechtsstaat if one does not already exist.

Whatever the place of morality in politics, Kant sees that humans are governed by their inclinations and desires, which make them partial to themselves and dangerous to one another. Further, actual rulers often repress their subjects. Yet, despite the fact that actual governments often fall short of realizing the principles of right, Kant abjures the idea that subjects ought to revolt against existing governments to create more perfect ones. He regards any “right to revolution” as incoherent because states are the only existing embodiment of right. Instead, Kant argues that subjects always have a duty to obey their governments, though they may use their public reason to criticize them.

Kant’s political philosophy is characterized by a disjunction between the realm of political principle and the material motives of much human behavior. In order to draw these two together, he argues that it is precisely by means of humankind’s negative or asocial characteristics that societies are created and drawn closer to meeting the requirements of morality. As he puts in his essay “Perpetual Peace,” the problem of civil government can be solved even for a race of devils, if they be intelligent. Even the most self-interested actors will come to understand that a state is the best means of protecting their own interests against others, even if they would rather exempt themselves from the law. They would design institutions which could constrain all to obey the law and act as if they were governed by morality. In Kant’s words, the establishment of a lawful and peaceful state “does not require that we know how to attain the moral improvement of men but only that we should know the mechanism of nature in order to use it on men, organizing the conflict of the hostile intentions present in a people in such a way that they must compel themselves to submit to coercive laws.”

Kant’s views on international relations exhibit the same tension between principle and fact. Kant argues that a state of perpetual peace is required morally. However, such a state can only come about when a set of improbable political conditions take effect. For perpetual peace to occur, all states must possess a republican civil constitution, participate in a union of states, abolish standing armies, and refuse to take on national debts for war, among several other conditions. Although we cannot expect existing governments to establish these conditions merely from their own desires, a historical teleology exists (Kant argues) whereby they might come about nonetheless. War plays a central role in this process. It is under the threat of war that humans form governments, and find that republican constitutions are most effective in meeting internal and external dangers. Moreover, as individuals and states pursue their interests through the medium of growing commerce, they find that war is incompatible with profit. States will thus avoid war in order more effectively to pursue wealth. Part of the reason that the continued pursuit of self-interest promotes peace is that modernization and economic advancement will make wars so catastrophic in their effects and expensive in their conduct that states will become increasingly inclined to avoid them. We therefore come closer and closer to the condition of peace that morality enjoins.

Although political institutions are brought about by the wicked elements in the human constitution, Kant hopes that such institutions might have some rehabilitative effects on their subjects. As he writes in “Perpetual Peace”: “A good constitution is not to be expected from morality, but, conversely, a good moral condition of a people is to be expected only under a good constitution.” However, the gap between the ideal world of morality and the natural world of politics can never be closed completely. Kantian morality depends on intentions. If a race of devils act according to the law only because they are compelled to by their own interest, their state would not be a morally good one. They only act as if they are moral. Morality requires that one follows duty out of a will to do so. Yet, it is impossible, within the natural world, to distinguish with certainty between an individual who acts from duty and one who follows the law out of a natural inclination. Indeed, it is impossible to make this distinction with certainty in one’s own case. Nor is it possible to distinguish a state of firmly established perpetual peace from a temporary lull in international conflict. In spite of these limits, Kant argues that the mere possibility of perpetual peace and of the coincidence of happiness and morality is enough to oblige us to make these ideals our ends.

For further introductory reading, see also:

Richard Velkley, Freedom and the End of Reason: On the Moral Foundations of Kant’s Critical Philosophy, Chicago: 1989.

Susan Meld Shell, Kant and the Limits of Autonomy, Cambridge: 2009.